材料一：班主任查阅了李晓同学小学期间个人成长档案，有这样一段评语：该生非常聪明，只是……因而学习成绩总是不理想。李晓同学也为此感到烦恼。

B

China has a very long history. It has many great traditional skills that have a widely influence(影响). Here we mainly talk about kites.

  The earliest mention of a kite in ancient China dates back to the 5th century BC. It was made of wood and used as a rescue signal(求救信号).

  At the time between 770 and 221 BC, Chinese kites were actually called “muyuan”. They were first invented in Shandong Province for military purposes(军事目的). The kites were used to send important messages during the Chu-Han War(203-202 BC). During the Tang Dynasty(618-907 AD), kites were made of lighter materials, such as bamboo, silk and paper. At that time, people began to fly kites for other purposes. It was believed that letting go of the kite could remove bad luck and illness.

  Today, kite-flying is an interesting and healthy activity in China. There are large kite-flying festivals held throughout the country, and kite makers can show off their skills during the festivals.

视网膜母细胞为恶性肿瘤，其发病与RB基因有关。RB基因编码的蛋白质称为Rb蛋白，分布于细胞核内，能抑制细胞增殖。正常人体细胞中含有一对RB基因，当两个RB基因同时突变产生突变蛋白时，会发生视网膜母细胞瘤。下列叙述正确的是

用代入法解方程组$\left\{\begin{array}{c}7x-2y=3\left(1\right)\\ x-2y=-12\left(2\right)\end{array}\right.$有以下步骤：

①：由（1），得y=$\frac{7x-3}{2}$（3）；

②：由（3）代入（1），得7x-2×$\frac{7x-3}{2}$=3；

③：整理得3=3；
④：∴x可取一切有理数，原方程组有无数个解
以上解法，造成错误的一步是（　　）